Many demodulators used in digital communication devices use zero-crossings to determine the polarity of the demodulated signal. In zero IF (Intermediate Frequency) applications, signal phase information can be extracted from the zero-crossings of the in-phase and out-of-phase, e.g., quadrature signals. These zero-crossings are detected by appropriate circuitry to demodulate the carrier signal and reconstruct the originally transmitted information signal.
In general, digital signals may be demodulated in various ways. In U.S. Pat. No. 4,322,851, issued Mar. 30, 1982, Ian Vance teaches a method of demodulating a binary FSK signal. This technique uses the direction of the phase rotation angle to detect signal polarity. A significant problem with this approach is its inability to modulate multi-level digital signals.
To overcome this limitation, a Cross-Differentiate-Multiply (CDM) demodulation technique may be employed. This technique is detailed in an article titled "An FM Detector for Low S/N" written by John Park and published in IEEE T-Comm, Vol. Com-18, No. 2, April, 1970. This technique is highly complicated and requires two differentiators, four multipliers, one divider, and two summers. The number of devices used, and hence, their current drain is very large. This makes the technique impractical for use, particularly in battery operated, portable communication applications.
Yet another approach employs the tan.sup.-1 (inverse tangent) operation {[q(t)]/[i(t)]} with digitized i and q signals. This technique requires two analog-to-digital converters and a processor. Once again the current drain is very high, particularly for use in battery powered, portable applications.
In a recently filed application entitled "Communication Device With Zero-Crossing Demodulator," Ser. No. 08/290,161, filed Aug. 15, 1994, now issued as U.S. Pat. No. 5,469,112 on Nov. 21, 1995 a method and device are described for demodulating multi-level digital signals. In that system there is provided phase axes generators, a zero-crossing detector, a phase angle estimator, and a decision device to use the i and q components of signals to demodulate even multi-level digitally modulated signals. The direction of the phase at zero-crossings and the phase rotation angles are used to estimate the contents of M-ary signals. In achieving its performance, a fixed reference (or hard-wired) synchronization clock is used. This fixed reference hard-wired clock has in the past been implemented in a simulation environment, and is not realizable in the field. Symbol synchronization, an equally important part of the demodulator, is, however, necessary. However, due to the nature of the detector, standard synchronization techniques, i.e., PLL (phase locked loop), or level crossing, cannot be employed.
A synchronization device and method is therefore desired which can be employed with the zero-crossing differential detector, and which exhibits little loss when compared to the use of the hard-wired clock.